Comment #1 on issue 286 by [email protected]: Inconsistency with repeated
fields using C++ protobufs
http://code.google.com/p/protobuf/issues/detail?id=286
Thanks for reporting this.
I believe what's happening here is some problems with Python refcounting
and the fact that the C++ implementation reuses submessages to avoid
reallocations.
keepitems = [r for r in p.repeated if r.name != 'B']
- this line ends up copying python objects wrapping the pointers to C++
objects stored in the repeated field of messages
del p.repeated[:]
- this line causes the C++ code to clear the repeated field. Under the
hood, the C++ objects are not deleted, but Clear()'ed. The implementation
is unaware that there are still python objects wrapping the C++ objects.
p.repeated.extend(keepitems)
- causes RepeatedPtrField::Add() to be called, which now returns the same
instances as were previously used.
In test1.py, keepitems contains a python object wrapping what's supposed to
be the 'A' object. When extend() is called, the same underlying C++ object
is used, causing the crash, so when it tries to copy the data over using
MergeFrom(), the crash is triggered.
In test2.py, keepitems contains a python object wrapping what's supposed to
be the 'B' object, except that C++ message has been cleared. When extend()
is called, the C++ object originally used to store the 'A' object is
returned. The MergeFrom() call succeeds, but since the message has been
cleared, you get an empty object.
I'm not familiar enough with Python and C extensions to propose a fix, or
even to know how the pure Python implementation handles this. Jisi, any
ideas on that or even just an owner for this bug?
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